The present invention relates to an optical fiber and an optical transmission line including the optical fiber, and more specifically an optical transmission line which can be used suitably in wavelength division multiplexing (WDM) optical communications.
As the optical transmission technique advances in terms of an increase in speed and capacity, the WDM transmission technique is attracting much attention as the mainstream technique. However, as the power of optical signal is enhanced, a new problem has started to occur, that is, a non-linear phenomenon which takes place due to the interaction between optical signals of two or more waves within an optical path.
Of the non-linear phenomenon, the four wave mixing (FWM) is considered to entail such a drawback that noise which occurs in the WDM transmission causes a serious adverse effect on the transmission, and therefore how to suppress it is being intensively studied. For example, OFC'94 Technical Digest PD19 proposes a dispersion shift optical fiber (DSF) which shifts the wavelength band to non-zero dispersion, as means for suppressing the non-linear phenomenon.
More specifically, such a DSF that has non-zero dispersion at a 1.55 μm-wavelength band is used. In this case, the absolute value of the non-zero dispersion (unit: ps/nm/km) is, in many cases, set to 0.5 to 5.
Further, the distortion of a waveform caused by self phase modulation (SPM) and cross phase modulation (XPM) is another very serious problem. In the studies on how to solve such a problem, a research of suppressing the non-linear refraction index (n2) reported in OFC'97 TuNlb or the like, is studied, and further attention is paid to the technique for enlarging the mode field diameter (MFD) of the DSF, that is, the technique for enlarging the effective core area (Aeff) of the core.
The distortion φNL of a signal, which is caused by the non-linear phenomenon is expressed generally by the following formula (1):
 φNL=(2π×n2×Leff×P)/(λ×Aeff)  (1)
From the formula (1), it is understood that in order to decrease the distortion, φNL of a signal, which is caused by the non-linear phenomenon, Aeff should be large to be advantageous. Further, Aeff is expressed by the following formula (2):Aeff=k×(MFD)2  (2)
where k is a constant.
From the formula (2), when MFD is large, a low non-linearity can be obtained very efficiently.
As reported in OFC'96 WK15 and OFC'97 TuN2, the enlargement of MFD is presently one of the most required characteristics for the DSF.
Besides the non-linear phenomenon, the distortion of waveform due to dispersion is another problem in terms of the transmission characteristics of optical fiber. For the suppression of the distortion of the waveform due to the dispersion while suppressing the non-linear phenomenon, the method for managing the dispersion over the total optical line is effective. For example, in Jpn. Pat. Appln. KOKAI Publication No. 6-11620 proposes an optical transmission line achieved by combining a single-mode optical fiber (SMF) having zero dispersion at about 1.3 μm and a dispersion compensation optical fiber (DCF).
Further, recently, an optical transmission line achieved by combining an SMF and a cable-type DFC is proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-325913.
In general, a DSF having zero dispersion or micro-dispersion at a 1.55 μm-wavelength band has a high non-linearity and is easily influenced by XPM or SPM. As in the conventional case, a great number of researches have been made to reduce the non-linearity by enlargement of the MFD of the DSF; however the enlargement of the MFD of the DSF generally entails bending loss or an increase in dispersion slope. In the case of a DSF having non-zero dispersion at a 1.55 μm-wavelength band, although its use at a wavelength band for zero dispersion is avoided, the wavelength dispersion per unit length (to be called local dispersion, hereinafter) is small, and therefore the FWM easily occurs as compared to the case of SMF.
On the other hand, the SMF has a larger positive local dispersion (about 16 ps/nm/km at a 1.55 μm-wavelength band) than that of a DSF having non-zero dispersion at a 1.55 μm-wavelength band, and therefore the FWM can be easily avoided. Further, since Aeff is relatively large (about 80 μm2), a non-linear phenomenon such as XPM or SPM does not easily occur. Here, the deterioration of a signal waveform occurs due to large dispersion at a 1.55 μm-wavelength band; however it can be solved by managing the total line with use of a dispersion compensation optical fiber such as described above. Further, in general, an SMF has a low loss and low PMD. That is, it can be said that an SMF is a relatively suitable fiber for the WDM transmission.
However, as the speed and capacity of data transmission is further increased in the future, very high power is input to a fiber and therefore even a present SMF might have a problem of non-linear phenomenon. Further, a dispersion compensation optical fiber for compensating dispersion of SMF has a high non-linearity due to its structure, and therefore a non-linear phenomenon such as XPM or SPM easily occurs.
Under these circumstances, an object of the present invention is to provide a positive dispersion optical fiber of a new type, which solves the above-described problems.
Another problem of the present invention is to provide an optical transmission line which includes such a positive dispersion optical fiber in its part.